In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. In particular, to provide a competitive alternative to conventional cinematic-quality projectors, an electronic or digital projection apparatus must meet high standards of performance, providing high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 1,000:1.
The most promising solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators. The first type of spatial light modulator is a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat. No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951 (Anderson); and U.S. Pat. No. 6,089,717 (Iwai). DMDs have been employed in digital projection systems. However, although DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, inherent resolution limitations (with current devices providing only 1024×768 pixels) and high component and system costs have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is a liquid crystal device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size and favorable device yields. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,221 (Maki et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.).
In an electronic projection apparatus using spatial light modulators, individual colors, conventionally red, green, and blue (RGB), are separately modulated in a corresponding red, green, or blue portion of the optical path. The modulated light of each color is then combined in order to form a composite, multicolor RGB image. There are two basic approaches for projection optics that combine the modulated color light. The first approach, which can be characterized as a convergent approach, is adapted from earlier, conventional projection systems. Using the convergent approach, the component red, green, and blue light have separate axes which are converged by separate projection optics that effectively direct and focus each light path as necessary in order to form a composite, multicolor color image at some focal plane. As an illustrative example, U.S. Pat. No. 5,345,262 (Yee et al.) discloses a convergent video projection system. Significantly, the disclosure of U.S. Pat. No. 5,345,262 illustrates one of the major problems with the convergent projection approach: namely, that the separate color images must be properly registered on the projection surface. Misregistration or poor focus along any one of the color light projection paths can easily result in an unsatisfactory image. It is instructive to observe that, using this approach, the image paths are converged only at the focus plane (screen).
U.S. Pat. No. 5,907,437 (Sprotbery et al.) discloses an attempt to simplify design complexity and alleviate some of the light path alignment and registration problems inherent to multicolor projection systems using the convergent approach described above. In the U.S. Pat. No. 5,907,437 disclosure, a light valve projection system is described in which a converging optical system converges the red, green, and blue modulated light paths in order to form an internal converged image, which is then re-imaged to the screen by the projection lens. The design strategy outlined in U.S. Pat. No. 5,907,437 thus simplifies the projection lens design task for a system using the convergent approach. However, other problems inherent to a convergent approach remain.
One notable problem with approaches similar to that disclosed in U.S. Pat. No. 5,907,437 is a relatively high etendue. As is well known in the optical arts, etendue relates to the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image. Numerically, etendue is proportional to the product of two factors, namely the image area and the square of the numerical aperture. Increasing the numerical aperture, for example, increases etendue so that the optical system captures more light. Similarly, increasing the source image size, so that light originates over a larger area, increases etendue and, therefore, brightness. As a general rule, increased etendue results in a more complex and costly optical design. Using an approach such as that outlined in U.S. Pat. No. 5,907,437, for example, lens components in the optical system must be designed for large etendue. The source image area for the light that must be converged through system optics is the sum of the combined areas of the spatial light modulators in red, green, and blue light paths; notably, this is three times the area of the final multicolor image formed. That is, for the configuration disclosed in U.S. Pat. No. 5,907,437, optical components handle a sizable image area, therefore a high etendue, since red, green, and blue color paths are separate and must be optically converged. Moreover, although the configuration disclosed in U.S. Pat. No. 5,907,437 handles light from three times the area of the final multicolor image formed, this configuration does not afford any benefit of increased brightness, since each color path contains only one-third of the total light level. In particular, the second relay lens and the projection lens of a convergent optics system such as that disclosed in U.S. Pat. No. 5,907,437 are inherently constrained by a large etendue, which adds cost and complexity to such a solution. Moreover, the second relay lens must be color corrected over the full visible spectrum. At the same time, different segments of the relay lens and of the projection lens handle different wavelengths, so that localized lens imperfections, dust, or dirt not only affect the projected image, but can impact the color quality. In light of etendue constraints, of color correction requirements, of dust and dirt sensitivity, and of the need for maximizing brightness levels for digital projection, there appear to be significant inherent limitations that hamper the convergent approach exemplified in U.S. Pat. No. 5,907,437.
An alternative approach to projection optics can be characterized as a coaxial approach. In contrast to the convergent approach in which component red, green and blue light beams are bent to converge at a focal plane, the coaxial approach combines the component red, green, and blue modulated light beams along a common axis. In order to do this, the coaxial approach employs a dichroic combining element, such as an X-cube or Philips prism. X-cubes or X-prisms and related dichroic optical elements, such as those disclosed in U.S. Pat. No. 5,098,183 (Sonehara) and U.S. Pat. No. 6,019,474 (Doany et al.) are well known in the optical imaging arts. The dichroic combining element combines modulated light from each color path and folds the color paths together along a common axis in order to provide the combined color image to a projection lens. Referring to FIG. 1, there is shown a simplified block diagram of a conventional digital projection apparatus 10 using the coaxial approach. Each color path (r=Red, g=Green, b=Blue) uses similar components for forming a modulated light beam. Individual components within each path are labeled with an appended r, g, or b, appropriately. For the description that follows, however, distinctions between color paths are specified only when necessary. Following any of the three color paths, a light source 20 provides unmodulated light, which is conditioned by uniformizing optics 22 to provide a uniform illumination. A polarizing beamsplitter 24 directs light having the appropriate polarization state to a spatial light modulator 30 which selectively modulates the polarization state of the incident light over an array of pixel sites. The action of spatial light modulator 30 forms an image. The modulated light from this image, transmitted along an optical axis Or, Og, Ob through polarizing beamsplitter 24, is directed to a dichroic combiner 26, typically an X-cube, Philips prism, or combination of dichroic surfaces in conventional systems. Dichroic combiner 26 combines the red, green, and blue modulated images from separate optical axes Or, Og, Ob to form a combined, multicolor image for a projection lens 32 along a common optical axis O for projection onto a display surface 40, such as a projection screen.
In contrast to the convergent approach outlined above with reference to U.S. Pat. No. 5,907,437, the coaxial approach, as shown in the block diagram of FIG. 1 and as exemplified in U.S. Pat. No. 5,808,795 has a number of advantages. With respect to light throughput, the coaxial approach, because it combines light paths along a common axis, does not increase the etendue of the optical system. Instead, with respect to projection lens 32, dichroic combiner 26, by folding the appropriate optical axes Or and Ob to join with optical axis Og and form a common optical axis O, optically overlaps the areas of spatial light modulators 30r, 30g, 30b. Thus, the etendue has no increase whether one, two, three, or more spatial light modulators are combined in this way. Since each light color is separately modulated, then combined and provided to projection lens 32 along a common optical axis O, no optical system is required between dichroic combiner 26 and projection lens 32.
A Philips prism, such as that disclosed in U.S. Pat. No. 3,202,039 (DeLang et al.) could alternately be employed as dichroic combiner 26. Familiar to those skilled in the digital image projection arts, Philips prisms have been employed as chromatic separator or combiner components in projector designs such as those disclosed in U.S. Pat. Nos. 6,280,035 and 6,172,813 (both to Tadic-Galeb et al.); U.S. Pat. No. 6,262,851 (Marshall); and U.S. Pat. No. 5,621,486 (Doany et al.), for example.
While digital projection apparatus 10 designed using the basic model of FIG. 1 are able to provide good levels of image quality, there is room for improvement. Constraints imposed by dichroic coatings are a key consideration. Dichroic coatings used within dichroic combiner 26 can be expensive and difficult to design and fabricate for suitable performance with incident light over a wide range of angles, particularly in projection applications where high brightness levels and a broad color gamut are needed. Dichroic coatings reflect and transmit light as a function of incident angle and wavelength. As the incident angle varies, the wavelength of light that is transmitted or reflected also changes. Where a dichroic coating is used with an optical system having a low F# and a broad spectrum, the typical result is a variable efficiency versus both angle and wavelength for both transmitted and reflected light. Misalignment or wedge of the interior surfaces of an X-prism type dichroic combiner can also cause image separation, image blur, and color shading. The “seam” at which dichroic surfaces are combined tends to appear as one or more linear shadow artifacts in the displayed image. Fabrication of a high-quality X-cube is further complicated by the requirement that individual component prisms have identical refractive indices; in practice, this is best accomplished when the same glass melt is used for all prism components
Generally these and other various problems that effect X-prisms can be overcome, and X-prisms are widely used in projection systems. However, obtaining high quality custom designed X-prisms can be a problem for prototype or low manufacturing volume applications. Moreover, high brightness applications such as occur in digital cinema systems, can impose high heat levels, which can damage adhesives and coating surfaces of the X-prism. Admittedly, the problems experienced with the dichroic separator (which sees intense light loads) and the dichroic combiner (which sees lower light loads, but through which the final image is formed) do not have the same sensitivities, but similar solutions may be needed by both. The design and fabrication of both the dichroic separator and combiner can be helped if the F# can be increased. In particular, if a larger F# light cone can be used, surface tolerance requirements in a dichroic combiner can be relaxed, thereby reducing cost and alignment complexity. However, conventionally, a light cone having a smaller F# is used in projection systems, since system designs are directed to maximizing brightness.
As is another well known principle applied in the design of projection apparatus, it is beneficial to minimize the retrofocus distance of projection lens 32, thus minimizing the working distance requirements and cost of projection lens 32. It would be preferable to avoid the cost and complexity requirements of a projection lens having a long back focal length relative to its effective focal length, such as the solution disclosed in U.S. Pat. No. 6,008,951 (Anderson), for example.
U.S. Pat. No. 5,944,401 (Murakami et al.) discloses, as an alternative to X-cube dichroics, a V-prism optical block comprising dichroic surfaces within plastic prisms. This solution provides some relief for back working distance requirements, since the refractive index of plastics exceeds that of air. To minimize back working distance, transmissive spatial light modulators are employed, allowing image-formation as close to the combining optical block as possible. However, this arrangement would not be well-suited for projector apparatus using reflective spatial light modulators, since back working distance requirements are still excessive. In terms of back working distance, the solution of U.S. Pat. No. 5,944,401 is not advantaged over conventional X-cube designs. A sizable projection lens would be required for full-scale cinema projection. Moreover, the solution disclosed in U.S. Pat. No. 5,944,401 does not address the inherent angular limitations of dichroic surfaces described above. Thus, brightness levels are constrained with this type of design solution.
U.S. Pat. No. 5,597,222 (Doany et al.) discloses, for use in a digital projector, an optical relay lens system that alleviates some of the difficulties noted above that relate to inherent tolerance problems and projection lens working requirements. U.S. Pat. No. 5,597,222 discloses the use of a single 1X, double-telecentric relay lens to relay the combined image from individual RGB color paths to a MacNeille polarizing beamsplitter (PBS), also termed a polarization beamsplitter. In U.S. Pat. No. 5,597,222 spatial light modulators are disposed very near a dichroic combiner X-cube, to minimize thereby some of the potential adverse effects of imperfections in outer surface flatness and tolerance errors in inner surface fabrication. The system disclosed in U.S. Pat. No. 5,597,222 is advantaged in that the design of its projection lens is simplified when compared with similar designs. The working distance requirements for the projection lens are significantly reduced using the design approach of U.S. Pat. No. 5,597,222. The single 1X double telecentric relay provides the necessary working distance to allow insertion of the MacNeille PBS prior to the intermediate internal combined image in the image path. The projection lens can then re-image this internal image to the screen without the requirements for long working distance that are typically required when using a PBS and/or a dichroic color combiner, such as an X-prism.
The solution presented in U.S. Pat. No. 5,597,222, however, falls far short of what is needed to compensate for inherent problems with X-cube coatings and surfaces so that both image brightness and color gamut can be maintained. For example, the design noted in U.S. Pat. No. 5,597,222 fails to address inherent angular dependencies in the dichroic coating response, so that it remains difficult to support a large color gamut while maintaining image brightness at the same time. Moreover, the projection lens must also use a high numerical aperture with this design, which implies added cost over designs with lower numerical aperture. Because of the scale of spatial light modulator components, the design of U.S. Pat. No. 5,597,222 is still very dependent on high-quality X-cube design. Further, the arrangement disclosed in U.S. Pat. No. 5,597,222 employs a relatively large number of optical components between a polarizing beamsplitter and its modulating LCD. With a large number of optical components in the path of a polarized illumination source, some unavoidable stress birefringence would necessarily alter the polarization states of both unmodulated and modulated light traveling in both directions, resulting in loss of image contrast.
U.S. Pat. No. 5,357,289 (Konno et al.) discloses a system that is similar to that disclosed U.S. Pat. No. 5,597,222, as it uses a single 1X relay lens to present an internal intermediate image to the projection lens, thereby significantly reducing the working distance requirements imposed on projection lens design. U.S. Pat. No. 5,357,289 provides an alternate construction to that shown in U.S. Pat. No. 5,597,222 for using polarization and color combining prisms. In the apparatus of U.S. Pat. No. 5,357,289, both the polarizing and color-combining prism are in the vicinity of the spatial light modulators, rather than spaced well apart, as in the apparatus of U.S. Pat. No. 5,597,222. Instead of the conventional X-prism, the apparatus of U.S. Pat. No. 5,357,289 uses a V-prism as a color combiner, where the V-prism is similar to that disclosed in U.S. Pat. No. 5,944,401 described above. The V-prism approach avoids some of the inherent problems with X-cube fabrication and use. While the approach disclosed in U.S. Pat. No. 5,357,289 eases the demands on projection lens design, the imaging relay (first lens group) presents a challenge, since it must provide a long working distance for the spatial light modulators and associated PBS and color-combining V-prism. As with the approach noted in U.S. Pat. No. 5,597,222, the approach shown in U.S. Pat. No. 5,357,289 uses a single imaging relay lens for all three colors (RGB), operating nominally at 1X magnification. As was seen with the U.S. Pat. No. 5,597,222 apparatus, the U.S. Pat. No. 5,357,289 approach requires a complex imaging relay lens that is fully color corrected over a broad part of the visible spectrum in order to form a white light image having minimal color aberrations or color differences in the third order aberrations, particularly with respect to distortion and defocus.
U.S. Pat. No. 6,247,816 (Cipolla et al.) discloses use of a 1X relay lens for relaying an intermediate image towards a dichroic combiner in only one of the color paths. The solution in U.S. Pat. No. 6,247,816 addresses a component packaging problem, but does not alleviate any of the angular constraints imposed by dichroic combiner response. Neither does the solution in U.S. Pat. No. 6,247,816 provide any relief with respect to back working distance requirements of the projection lens.
U.S. Pat. No. 4,836,649 (Ledebuhr et al.) discloses a projector system that uses internal imaging in both the illumination and imaging paths. The system utilizes nominally 1X imaging relays, with the portion of the relays used to interface to the light valves is common to both the illumination and imaging paths. The cited advantages of this architecture are to minimize the size of polarization components and to help alleviate back working distance constraints for the projection lens. While this arrangement provides some advantages, the color-combining dichroic surfaces must still handle light at low F# values, resulting in reduced color gamut. Likewise, as the imaging relay directly provides an internal white light image, the imaging relay lens must be color corrected across the entire visible spectrum. Finally, the projection lens must also operate at a low F# when using this solution.
Thus, it can be seen that there is a need for improvement in digital projection optics design that alleviates the inherent angular limitations of dichroic coatings while providing maximum brightness and color gamut, minimizes the working distance requirements of projection optics, and allows a high F# for projection optics.
An optical system described in pending U.S. patent application Ser. No. 10/050,309 provides an optical configuration that alleviates many of the problems described previously. This system provides for a projection system that produces an internal white light image, by means of combining three color (RGB) intermediate images relayed by three optical systems and combined by a color combining prism. This system provides working distance advantages, as well as a reduced numerical aperture at the color combining prism, which improves the manufacturability of the prism. Although this system can be configured with various components, it provides superior performance when the color combining prism is a V-prism (similar to the V-prism described in U.S. Pat. No. 5,357,289) and the polarization beamsplitter is a wire grid polarizer (see U.S. Pat. No. 6,243,199 (Hansen et al.)). Although this system works remarkably well, including for high brightness projection applications such as digital cinema, there are opportunities both for improvements and extensions. In particular, the system of the present invention provides opportunities to have smaller and less expensive optical components, an easier opto-mechanical package, and a less difficult optical design. Other advantages will become apparent from the discussion of the system of the present invention.